In summary, our overall program has focused on the regulation of delayed type hypersensitivity in mice. We have found that antigen presenting cells are crucial in the induction of these responses; that histamine provides a potent microenvironmental influence by which the reactions can be regulated and that release of serotonin by mast cells is crucial to these reactions and proceeds in a T cell dependent manner by a unique and, heretofore, unrecognized mast cell release mechanism.